U.S. patent application number 13/868803 was filed with the patent office on 2014-09-25 for bucking circuit for annulling a magnetic field.
This patent application is currently assigned to VALE S.A.. The applicant listed for this patent is VALE S.A.. Invention is credited to Benjamin David Polzer, Peter Whyte Walker, Gordon Fox WEST.
Application Number | 20140285206 13/868803 |
Document ID | / |
Family ID | 50478115 |
Filed Date | 2014-09-25 |
United States Patent
Application |
20140285206 |
Kind Code |
A1 |
WEST; Gordon Fox ; et
al. |
September 25, 2014 |
BUCKING CIRCUIT FOR ANNULLING A MAGNETIC FIELD
Abstract
A method and apparatus is provided for bucking a magnetic field
of known geometry and time variation by means of a plurality of
bucking loops. It utilizes multiple loops, each of which is
energized by an electric current that creates a magnetic field of
the known time variation. The multi-loop field forms a bucking
magnetic field that better opposes the spatial variation in the
known magnetic field over a volume than can the magnetic field from
a single loop. The present invention is useful in electromagnetic
measurements, where the magnetic field of a controlled source
transmitter must be annulled at a magnetic field sensor. It is
particularly useful for cases where the magnetic sensor may move
relative to the transmitter, such as in certain airborne
electromagnetic measurements.
Inventors: |
WEST; Gordon Fox; (Toronto,
CA) ; Walker; Peter Whyte; (Mississauga, CA) ;
Polzer; Benjamin David; (Sudbury, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VALE S.A.; |
|
|
US |
|
|
Assignee: |
VALE S.A.
Rio de Janeiro
BR
|
Family ID: |
50478115 |
Appl. No.: |
13/868803 |
Filed: |
April 23, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61804080 |
Mar 21, 2013 |
|
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Current U.S.
Class: |
324/333 ;
324/339 |
Current CPC
Class: |
G01V 3/38 20130101; G01V
3/165 20130101; G01V 3/30 20130101; G01V 3/26 20130101; G01V 3/28
20130101; G01V 3/18 20130101; G01V 3/107 20130101 |
Class at
Publication: |
324/333 ;
324/339 |
International
Class: |
G01V 3/28 20060101
G01V003/28 |
Claims
1. A bucked transmitter comprising: a transmitting electrical
circuit comprising a transmitter loop; a transmitter loop framework
to support said transmitter loop, said transmitter loop framework
being substantially rigid; a transmitter adapted to energize said
transmitting circuit with a transmitting electric current having a
known waveform so as to form a primary magnetic field; a plurality
of separate, substantially planar bucking loops, each being
dimensionally smaller than said transmitter loop, arranged
coaxially along an axis substantially parallel to the local
direction of said primary magnetic field and energized with current
by a current controller; a substantially rigid support to which
each of said bucking loops is affixed, said support being attached
said transmitter loop framework, wherein said bucking loops are
stably located with respect to geometrical aspects of said primary
magnetic field; a nulling axis which extends bi-directionally in a
perpendicular direction from a plane parallel to said bucking loops
to a termination point at each end of said nulling axis; wherein a
bucking magnetic field is formed within a bucked volume by current
in said bucking loops, said bucked volume being substantially
centered on said nulling axis, said termination points are located
where the bucking magnetic field fails to substantially annul said
primary magnetic field, said bucking magnetic field is
substantially in a direction opposite to, and is substantially
equal in magnitude with, said primary magnetic field, such that
said bucking magnetic field substantially annuls said primary
magnetic field within said bucked volume.
2. The bucked transmitter according to claim 1 wherein said
plurality bucking loops comprise a first bucking loop and a second
bucking loop, said first and second bucking loops being
substantially circular.
3. The bucked transmitter according to claim 1 wherein said first
bucking loop and said second bucking loop are arranged in a common
plane, said first bucking loop being geometrically smaller than
said second bucking loop.
4. The bucked transmitter according to claim 3 wherein said common
plane comprises the plane of said transmitter loop, the magnetic
moments of said first bucking loop and said transmitter loop are
substantially parallel in the same direction, and the magnetic
moments of said second bucking loop and said transmitter loop are
substantially opposed.
5. The bucked transmitter according to claim 2 wherein said first
and second bucking loops are each disposed with the same effective
radius, number of turns and with magnetic moments in the same
direction, wherein each of said bucking loops is offset from the
transmitter plane in the direction of the common axis.
6. The bucked transmitter according to claim 5 wherein said bucking
loops are coaxial with said transmitter loop, said first and second
bucking loops being offset from the plane of the transmitter loop
by equal distances and in opposite directions, and wherein the
magnetic moments of said first bucking loop, said second bucking
loop are substantially in parallel and are opposed to the magnetic
moment of said transmitter loop.
7. The bucked transmitter of claim 1, wherein the electrical
connections supplying electrical current to the loops are selected
from the group consisting of: bifilar wires, coaxial wires and
combinations thereof.
8. The bucked transmitter of claim 1, wherein the transmitter loop
and the bucking loops are disposed to form a series electrical
circuit, whereby the current controller for the bucking loops is
the transmitter.
9. The bucked transmitter of claim 1, wherein a bucking loop and
its current controller form a first electrical circuit and the
transmitter loop and the transmitter form a second electrical
circuit.
10. The bucked transmitter of claim 1 comprising a current sensor
and data recorder, wherein the current in said transmitter loop is
measured and recorded.
11. The bucked transmitter of claim 1 comprising a current sensor
and data recorder, wherein the current in a bucking loop is
measured and recorded.
12. The bucked transmitter according to claim 1 comprising a
sensing means responding to the geometry of the loops and a data
recorder, wherein said sensing means is disposed to respond to the
shape and positions of the loops, and said data recorder records
data output by said sensing means.
13. An electromagnetic measurement apparatus, comprising: the
bucked transmitter of claim 1; a magnetic field sensor; a receiver
adapted to controlling said magnetic field sensor so as to permit
the output of said sensor to be measured and recorded; a data
recorder disposed to record data from said receiver; a sensor
support frame comprising a mechanical support apparatus adapted to
support and limit the motion of said magnetic field sensor to the
bucked volume of said bucked transmitter, whereby said sensor
support frame is affixed to the transmitter loop framework of said
bucked transmitter; wherein the location of said bucked volume is
substantially fixed relative to said mechanical support
apparatus.
14. The electromagnetic measurement apparatus of claim 13,
comprising means to measure the geometry of said magnetic field
sensor with respect to said transmitter loop and said bucking
loops, whereby said geometrical data are recorded so as to permit
calculation of the magnetic field absent bucking, the primary
magnetic field, the bucking magnetic field, and combinations
thereof, at the magnetic field sensor.
15. The electromagnetic measurement apparatus of claim 13,
comprising: a carrier to transport said electromagnetic measurement
apparatus, a means of transporting said carrier, a sensor to detect
the position of said carrier, and a means of recording said
positions.
16. The electromagnetic measurement apparatus of claim 13, wherein
the means of transporting the carrier is selected from the group
consisting of aircraft, airships, dirigibles, helicopters, towed
birds, ground vehicles, towed trailers, barges, ships, boats,
submersible vehicles and combinations thereof.
17. The electromagnetic measurement apparatus of claim 13 adapted
for helicopter borne geophysical surveying, wherein said apparatus
is suspended on a tow cable below a helicopter and the plane of the
transmitter loop is substantially horizontal.
18. The electromagnetic measurement apparatus of claim 13
comprising an orientation sensor, whereby the orientation of the
magnetic field sensor is measured and recorded.
19. The electromagnetic measurement apparatus of claim 13
comprising an orientation sensor, whereby the orientation of the
transmitter loop is measured and recorded.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the invention
[0002] Aspects of this invention relate generally to bucking
systems and to methods of substantially cancelling a magnetic field
at points within a volume. More particularly, aspects of this
invention may be used in electromagnetic prospecting to cancel the
effect of a large transmitted field on a magnetic field sensor
without appreciably modifying the interaction of the transmitted
field with the ground. The current invention facilitates such
cancellation when the sensor is displaced relative to the
transmitter.
[0003] 2. Description of the Related Art
[0004] Electromagnetic ("EM") exploration methods comprise an
important part of the geophysical methods used to map the Earth in
the search of oil, gas and mineral deposits, aquifers and other
geological features. EM methods can broadly categorized into two
categories, passive source methods in which an electromagnetic
survey apparatus is used to map naturally occurring time-variations
of the electromagnetic fields over the surface of the Earth, and
active source methods, in which the electromagnetic field is
emitted from a transmitter that is an integral part of the survey
apparatus.
[0005] Active source EM systems comprise several parts; a
transmitter and antenna to create an electromagnetic field, a
sensor and a receiver to detect the signal from the transmitter,
and related electronics, mechanical elements, data recorder and a
power source. Although EM systems also comprise passive systems in
which the natural variation of the electromagnetic field is
measured absent a transmitter, in the following discussion, EM
systems shall be understood to comprise only those systems with a
transmitter unless otherwise noted.
[0006] Active source EM systems operate by supplying a time varying
current waveform to a transmitter coil, or loop, which creates a
corresponding "primary" time varying magnetic field. Time
variations in the primary field then induce eddy currents in the
Earth, resulting in "scattered" magnetic fields. The scattered
fields, together with the primary field, are measured with a
receiver, usually by employing a coil, loop or magnetometer sensor.
Characteristics of the scattered magnetic field may then be used to
determine the electrical properties of the ground. These properties
may then be used as a basis for geological interpretation such as
inferring the presence of geological features. For example, the
characteristic of the scattered field that is in-phase with the
primary field may be of interest for detecting highly conductive
ores. Improving the characterization of the scattered magnetic
field leads to improved geological inferences and hence to the
success of any prospecting venture employing an active source
system.
[0007] In the following, "coil" and "loop" may be used to mean the
antenna through which the primary field is emitted, and either may
comprise one or more windings (turns) of electrical conductor. The
resulting magnetic fields are then detected with a receiver that
includes one or more magnetic field sensors. A magnetic field
sensor may be a coil, loop or circuit element in which changes in
the magnetic flux density are detected in accordance with Faraday's
Law, or it may be a magnetometer. Examples of magnetometers include
devices that employ fluxgate, feed-back coil, Hall effect, and
optically pumped atomic vapor principles for detecting the magnetic
field, as well as related instruments.
[0008] Loops and coils may comprise circular, elliptical, oval,
helical or other similar rounded shapes, or sections thereof, and
may comprise linear segments which together form a closed shape,
usually with internal angles of less than 180 degrees, examples of
which are rectangles, hexagons, octagons, dodecagons and so forth.
Loops comprise at least one conductive winding, generally composed
of an electrically conductive substance such as copper or aluminum,
but may comprise a superconductor. Loops fashioned as convex
symmetric polygonal shapes with a plurality of sides may be
considered to be substantially circular, as would a circular
loop.
[0009] When an EM system is deployed in the air, one of two
configurations are usually employed. In the first configuration,
the transmitter and the receiver may be located on the same
platform, structure or "carrier", while in the second
configuration, the receiver may be towed at some distance behind
the transmitter. In the first configuration, the transmitter and
the receiver may be mounted on an aircraft "carrier", examples of
which include the system once operated by the Geological Survey of
Finland and the Hawk system built by Geotech Ltd. It is also
possible to mount the transmitter and receiver on a platform or
chassis "carrier" which is towed from the aircraft. Such carriers
are generally towed beneath helicopters, and are often referred to
as "birds", "sondes" or "bombs". In such cases, the bird may be
typically towed 30 to 60 meters below the helicopter at altitudes
of about 30 to 60 meters above the ground. In such systems, because
the transmitter and receiver are located in close proximity, the
primary field at the receiver may be orders of magnitude larger
than the scattered field.
[0010] When the primary field is much larger than the scattered
field, a means of primary- scattered field separation is required
to permit accurate detection of the much smaller scattered field.
One common method of accomplishing this is by time separation,
whereby the primary field is broadcast as a series of shaped pulses
with alternating polarity, with each pulse separated by an off-time
during which no current flows in the transmitter loop. If the
scattered fields are measured during this off-time, the primary
field will not be present and highly sensitive measurements of the
scattered field are possible. The disadvantage of limiting
measurement to the off-time is a loss of information. In
particular, the in-phase component of scattered response may be
poorly rendered, with the result that certain highly conductive
ores may be undetectable. Since highly conductive ores are often
targeted in airborne electromagnetic ("AEM") surveys, accurate
on-time measurements may be quite important to the success of AEM
ventures. It is therefore advantageous to acquire good quality
in-phase AEM data.
[0011] A number of AEM systems have used off-time measurements as a
means of separating the scattered field from the primary field.
Most notable of these was the Barringer Input system and the
systems derived from it such as Geotem, Megatem and Questem.
[0012] Bucking provides an alternative means of primary-scattered
field separation. When the in-phase component of the primary field
is large, such as when the transmitter and the receiver are located
in close proximity, a bucking loop may be used either to directly
cancel the primary field at the receiver through active bucking, or
to cancel its effect on the receiver through passive bucking.
Active bucking involves the creation of a bucking magnetic field
that will substantially cancel the primary field seen by the
magnetic field sensor of the EM system. Usually the bucking
magnetic field is created by passing the time varying current
waveform used to energize the transmitter loop or antenna through a
second smaller loop that is near the magnetic field sensor. In
passive bucking, an additional magnetic field sensor is used to
detect a different combination of primary and scattered fields than
is seen by a single magnetic field sensor. The signals from the two
sensors are then combined in a way to annul the primary field in
the combined signal. Bucking may therefore be used advantageously
to acquire good quality in-phase AEM data in the presence of a
large primary field.
[0013] An additional advantage to bucking results as a consequence
of suppressing the primary field in the presence of the receiver.
When the primary field is bucked, the receiver may be operated with
higher sensitivity than were the field to be unbucked. More subtle
scattered field anomalies may therefore be detected, so permitting
the detection of smaller geological features with weaker physical
property contrasts without saturating the receiver.
[0014] Examples of systems using bucking are the Dighem helicopter
frequency domain system which employs passive bucking, as did a
proposed system by Whitton (US patent application 2003169045A1);
and the VTEM (US patent application 2011/0148421 A1) and the
Aerotem helicopter time domain systems that employ active
bucking.
[0015] In systems employing active bucking, the objective is to
annul the primary field at the receiver without appreciably
affecting the eddy current induction caused by the transmitter in
the Earth. Accordingly, the bucking loop is chosen to be
geometrically smaller than the transmitter loop but closer to the
sensor. As a result, the range of receiver positions over which the
field may be bucked is usually also small. Because of this, any
relative displacement of the magnetic field sensor with respect to
those loops may strongly affect the degree to which the primary
field is cancelled at the sensor. Accordingly, in the current state
of the art, the quality of the bucking improves as the system
becomes increasingly rigid.
[0016] An advantage of active bucking is that the primary field in
the vicinity of the receiver is suppressed, despite the fact that
the field is not perfectly cancelled at all nearby locations. In so
doing, eddy current induction due to changes in the primary field
within any metallic components of the receiver and its chassis is
strongly reduced.
[0017] In the current state of the art, bucking has been most
effective when the relative geometries of the transmitter loop, the
magnetic field sensor and the bucking loop are nearly rigidly
fixed. Whenever the loop geometries change either in shape or in
position relative to one another, unbucked residuals of the primary
field will appear as signals in the receiver. The residuals are
generally indistinguishable from the in-phase scattered field, and
so may degrade the quality of the measured scattered response. The
AEROTEM and Dighem systems employ a nearly rigid geometry, and so
minimize the variation in unbucked primary field residuals caused
by loop motion. Nevertheless, some unbucked residuals may occur,
even in a system with a nominally nearly rigid geometry. These
residuals may result from small changes in loop geometry, often
attributed to thermal expansion, producing a phenomenon known as
"drift".
[0018] Despite the advantages of a rigid geometry for accurate
bucking, and so for measuring the in-phase component of the
scattered field accurately, it may be necessary or advantageous to
permit some variation in the relative geometry of the transmitter
loop, the magnetic field sensor, and the bucking loop. The VTEM
system is illustrative of an AEM system that is substantially rigid
in the EM acquisition band, yet has a flexible geometry. The light
weight of its transmitter chassis permits a larger transmitter loop
and therefore moment than would be possible were the system to be
nearly rigid. Because the transmitter loop is deformable, it can be
handled with greater ease during lift-off and set-down stages of
each flight. Construction of the loop chassis in sections
facilitates transportation and breakage is easier to repair:
Collisions do not involve the catastrophic loss of a single rigid
chassis with its high-value components. The trade-off introduced as
a result of increased flexibility is that the fidelity of the
bucking is less than could be provided by a comparable nearly rigid
system.
[0019] Polzer et al (international patent application WO
2011/085462 A1) has noted a second advantage to allowing some
flexibility in the geometry of the transmitter loop, the magnetic
field sensor and the bucking loop . Polzer noted that the rotation
of an EM sensor in the background magnetic field of the Earth,
particularly in the 1-25 Hz low frequency range, creates noise
which had previously prevented the acquisition of high-precision
airborne electromagnetic data in that band. By employing a
stabilization system for motion isolation in which the magnetic
field sensor moves relative to the bird in which is housed,
high-precision airborne electromagnetic data in the 1-25 Hz band
may be acquired. In so doing, the geometry of the AEM system must
be flexible.
[0020] Thus, in the current state of the art in AEM surveying,
single loops are used to buck the primary field. Nearly rigid
systems provide relatively stable bucking and permit precise
in-phase measurements of the scattered field, sacrificing
transmitter moment, light weight and certain logistical advantages.
Flexible systems permit a larger transmitter moment and logistical
advantages, but with less perfect bucking, and less accurate
measurement of the in-phase component of the scattered field as a
consequence. Less accurate in-phase measurements may result in
poorer resolution of highly conductive geological features, many of
which are targets of EM surveys commissioned for mining
exploration. Less perfect bucking could also mean that larger
magnetic field amplitude variations may be encountered than in the
case of a well bucked system, and that as a consequence, EM data
may be acquired with lower resolution.
[0021] Bucking coils are not necessarily used with the intention of
annulling the field of a transmitter. For example, US Patent
application 2011227578 A1 to Hall et al describes an induction
logging tool which uses multiple bucking coils to redirect the
field produced by the transmitter at any angle from the rotational
axis of the logging tool.
[0022] Miles et al, in U.S. Pat. No. 7,646,201 B2, disclosed an AEM
system having a rigid transmitter loop concentric with an inner and
an outer receiver loop. By null coupling the receiver loop to the
transmitter, the receiver loop could be made mainly sensitive to
scattered field of the Earth generated within the annulus defined
by the receiver loop.
[0023] Kuzmin et at (Patent application US 2010/0052685) disclose
an active bucking system for the VTEM AEM system which has a
flexible geometry. The system consists of an outer transmitter loop
and an inner, coplanar and concentric bucking loop, both of which
are centered on a receiver loop. The bucking loop and the
transmitter loop are connected in series so that the primary field
at the receiver is approximately annulled. However, flexure in the
loop geometry causes shifts in the measured fields resulting from
unbucked residuals of the primary field at the sensor. In the case
of systems such as Kuzmin's where the transmitter and bucking loops
are approximately concentric around the magnetic field sensor, the
axial magnetic field, H.sub.z, though the center of each loop may
be computed, to good approximation, from:
H.sub.z(z)=i/{2*a*(1+(z/a).sup.2).sup.3/2)
[0024] where i is the current in the loop, a is the radius of the
loop and z is the offset on the axis through the loop.
[0025] It would be advantageous, in the case of AEM systems, if the
bucking apparatus could be designed so as to accommodate relative
motions of the transmitter loop, magnetic field sensor and bucking
loops so as to retain the advantages of system flexibility as is in
the case of the VTEM system while improving the bucking within a
volume defined by the motion of the magnetic field sensor relative
to the transmitter and bucking loops. Such a bucking apparatus
would be advantageous in flexible EM systems, and would improve
bucking in AEM systems employing motion isolation, as in the case
of Polzer's system. A first advantage of such a bucking apparatus
would be in yielding improved in-phase EM data, and so improved
sensitivity to highly conductive ores . A second advantage would be
in yielding data which may be acquired with improved resolution,
resulting in greater sensitivity to subtle features in the
scattered electromagnetic field.
DESCRIPTION OF THE INVENTION
[0026] The present invention improves the quality of the bucked
primary field over the current state-of the-art single loop bucking
where a magnetic field sensor may vary its location within a
defined volume located relative to the transmitter loop. The
present invention may also improve the quality of the bucked
primary field where the position of a transmitter loop, or parts of
it, may vary in relation to a bucking loop.
[0027] The present invention uses a plurality of coils or loops
which are energized with electrical current to create a "bucking
field" which substantially opposes the primary field on the
magnetic field sensor. By employing a plurality of loops,
substantial cancellation of the primary field may be effected over
a larger volume than may be accomplished with a single loop. In so
doing, the bucked field is less sensitive to changes in the system
geometry than when a single bucking coil is used.
[0028] By using a plurality of loops in a "bucking loop
arrangement", the geometrical variation of the primary field at the
sensor may be approximately matched (and opposed) over a larger
volume than is possible with a single loop. Since the loops used to
buck the primary field are not geometrically identical to the
transmitter loop, the range of volume over which the field is
bucked, and the degree to which the primary field is cancelled,
will depend on the particular application to which the present
invention is being applied. The purpose of the present invention is
not to identically cancel the primary field at all points in a
volume of interest, but to substantially oppose the primary field
on the magnetic field sensor over a certain volume in comparison to
what may be accomplished with a single bucking loop.
[0029] As a matter of definition, the words "cancel" and "nulling"
and "annulling", and variations thereon, refer to the effect of
substantially diminishing the primary magnetic field of a
transmitter loop over a volume of interest. The specific amount of
cancellation, and the volume over which the cancellation is to
occur, is understood to be determined by the requirements of the
specific method or apparatus that may utilize the present
invention.
[0030] It is noted that in the case where bucking loops
substantially cancel a magnetic field along an axis, the
cancellation extends radially from that axis as a result of the
divergence free property of the magnetic field if it is not of very
high frequency. Thus, where substantial cancellation occurs along
such a nulling axis, the existence of such a "nulling axis" implies
substantial cancellation within a volume comprising said nulling
axis. As a matter of definition, such a volume comprising a nulling
axis is defined to be a "bucked volume".
[0031] In the present invention, a set of multiple loops creates a
bucking field which oppositely matches the shape of the primary
field, namely its amplitude, polarity, time and spatial variation,
over a substantially fixed volume in space relative to the
transmitter antenna. Bucking will be more effective for a given
number of loops when the variation of the primary field over the
bucked volume is small.
[0032] By so matching the effect of the bucking loops to the effect
of the transmitter along a nulling axis, rather than at a point as
in the case of a single loop, substantial cancellation may be
obtained as the magnetic field sensor moves with respect to the
transmitter within a bucked volume substantially centered on such a
nulling axis.
[0033] Small changes in the transmitter loop geometry may cause
small shifts in the space occupied by the bucked volume. Provided
the geometry of transmitter loop is substantially rigid, such that
these shifts are small, a magnetic field sensor with a limited
range of motion will remain within the bucked volume. In airborne
electromagnetic surveying, for example, substantial rigidity may be
supplied either by composite structural members which support the
loops, or by combinations of such structural members and cables,
either of which may be used as a supporting framework in the
current state of the art, while structures which are nearly rigid
are generally constructed as shells from composite materials.
[0034] A nearly rigid structure has less flexibility than a
substantially rigid structure, and references to substantially
rigid structures herein are understood to include structures which
are nearly rigid.
[0035] The present invention may therefore be used to improve the
quality of the bucked primary field in cases where the geometry of
the magnetic field sensor or the transmitter loop varies in
relation to the location of the bucking loops in comparison to that
which may be attained using a single bucking loop.
[0036] The degree of cancellation and the size of the bucked volume
over which bucking may be achieved depends on the number of loops
used to oppose the primary field. For example, bucking loops may be
configured to match gradients in the primary field, and for to
match the curvature in the primary field, and so forth in analogy
to a Taylor series. Bucking loops may take the form of
superpositions of the aforementioned loop sets, depending on the
desired amount of primary field cancellation and the volume over
which the field is to be cancelled. The loops may be aggregated to
have the same effect as the aforementioned Taylor series without
being configured to simulate the individual terms of that
series.
[0037] To achieve satisfactory bucking over a bucked volume, the
selection of the effective bucking loop parameters must be chosen
carefully in order to provide a satisfactory result in which the
primary field will be suitably cancelled. Each bucking loop will
have a number of turns, an effective radius and a current which,
when aggregated, will form a bucking magnetic field with a
particular geometrical variation. By selecting the turns, radius
and current parameters carefully, a bucking circuit arrangement may
be designed to substantially annul the primary magnetic field over
a particular volume. Where the bucking and transmitter loops are
arranged in series, current is not a free parameter in the bucking
design and suitable combinations of turns and radii must be
carefully selected.
[0038] The plurality of bucking loops are mounted on a bucking loop
support. Better cancellation is achieved as the bucking loop
support becomes more rigid, and by ensuring this support is stably
located with respect to the geometrical aspects of the primary
field. The transmitter loop and bucking loops may be nearly, or
substantially, rigidly joined together, while permitting the
magnetic field sensor to move within the bucked volume.
[0039] The present invention may achieve effective cancellation
over the bucked volume by employing several configurations. For
example, a bucking loop may be configured to buck a primary field
in a number of ways, including: [0040] a) direct series connection
with the transmitter circuit, [0041] b) inductive connection with
the transmitter circuit, such as with a transformer, [0042] c) as a
circuit electrically independent of any transmitter circuit, apart
from a digital or analogue control signal, which may link the
two.
[0043] In cases where the bucking loops are in a series connection
with the transmitter loop, ignoring capacitive effects, the current
in each turn of the bucking loops will be substantially the same as
the current in the turns of the transmitter loop. Where the loops
are connected in a series connection, capacitances may cause high
frequency (or rapidly changing) components of the currents in some
bucking loops to differ from that in the transmitter loop current,
affecting the degree to which the primary field can be effectively
annulled.
[0044] In the field of airborne electromagnetic surveying, the AEM
system often comprises a large substantially circular transmitter
loop with a magnetic field sensor located in the plane of the loop.
In such cases, it may be advantageous to annul the primary field
throughout a volume around the nominal location of the sensor. In
systems where the magnetic field sensor is located in the center of
the transmitter loop, the bucking loop arrangement may comprise two
substantially circular bucking loops, with the bucking loops
arranged coaxially in the plane of the transmitter loop, referred
herein to as a "coplanar" configuration. In such a two bucking loop
coplanar configuration, the radius the inner bucking loop is
smaller than the radius of the outer bucking loop and the radius of
the outer bucking loop is smaller than the radius of the
transmitter loop. Furthermore, the sense of the magnetic moment of
the inner loop will be parallel to the magnetic moment of the
transmitter loop, while the magnetic moments of the inner and outer
bucking loops will be in opposition.
[0045] In a second example of a bucking circuit arrangement,
referred to herein as a "pseudo-Helmholtz" arrangement, the
plurality of loops may comprise at least one set of substantially
circular bucking loops arranged coaxially to a substantially
circular transmitter loop, but in planes offset in opposite
directions from the plane of the transmitter loop, and such that
the radius of the bucking loops is smaller than the radius of the
transmitter loop. In the coaxial configuration, the sense of the
magnetic moment of the bucking loops will be in opposition to the
magnetic moment of the transmitter loop.
[0046] In both the above cases, the nominal receiver location is at
the center of the transmitter loop, and the transmitter loop
generates a field on the loop axis that has a vanishing axial
gradient in intensity. The pair of bucking loops are therefore
configured to cancel both the axial field and the axial curvature
of the transmitter loop near the loop center. Where this is not the
case, the second bucking loop may be configured with the first to
cancel the field and its gradient, or three bucking coils may be
employed to cancel the field and its gradient and its
curvature.
[0047] The present invention has application to the field of
airborne electromagnetic surveying in which a small field scattered
from the Earth is measured in the presence of a large primary
field. In the current state of the art, single bucking loops are
employed to annul the primary field at a receiver. However, as
larger transmitter moments and more precise and lower frequency
scattered field measurements become feasible, improved means of
measuring the scattered field in the presence of a large field
which accommodates a varying transmitter-receiver geometry are
required. In particular, an important aspect of acquiring scattered
field measurements at sub 25-Hz frequencies is in permitting a
flexible transmitter-receiver geometry, as elucidated in patent WO
2011/085462 A1 to Polzer.
[0048] The present invention may also advantageously improve
scattered field measurements where a flexible transmitter-receiver
geometry is present by increasing the effective volume over which
the primary field may be annulled, and there are a number of ways
the present invention may be advantageously employed to this
effect. As one example, where a motion isolation system is employed
to permit acquisition of sub-25 Hz electromagnetic data, the
transmitter loop and a plurality of bucking loops may be joined
together, where the bucking loops are connected to a housing
containing a motion isolation system in which a magnetic field
sensor is mounted. This aspect of the invention has the advantage
of permitting the receiver to move independently of its chassis,
but within the bucked zone of the primary field, and so is suitable
for use in motion isolation devices such as the one disclosed in
patent WO 2011/085462 A1. Another advantage of the present
invention is that eddy current induction due to the primary field
in a volume surrounding the receiver is suppressed in comparison to
that of a single bucking loop, so decreasing system noise. The
present invention may be also advantageously employed in
flexible-geometry EM systems, such as may be exemplified by the
VTEM system, where the receiver and its bucking loop may be mounted
at the center of a substantially circular, flexible transmitter
loop, whereby additional bucking loops would diminish the
variability of the bucked field due to the relative motions and
flexure in the loops, and a plurality of bucking loops may lie in
the same plane as the transmitter loop. In such a flexible system,
variation in the bucked field may be reduced by adding one or more
additional bucking loops to the plane of the transmitter.
[0049] The foregoing examples have illustrated various possible
uses of the present invention systems with a flexible geometry.
However, the present invention may also be advantageously employed
in so-called rigid geometry AEM systems, in which the geometry of
the transmitter, bucking and receiver loops are nearly rigidly
mounted with respect to one another, an examples of which is the
Aerotem system. Such systems may experience drift, a component of
which may be due to small geometrical variations, caused for
example by small dimensional changes in the rigid supports. The
effect of such changes may be diminished with the present invention
by permitting the size of the bucked volume to be increased, and
thus reducing the degree rigidity required Since the rigidity
requirement in airborne systems limits the size of the system due
to the weight of the rigid supports, the present invention may be
advantageously employed in such rigid systems by either permitting
the system to be lightened with less rigidity, so reducing the
operating costs, or by extending the dimensions of the system, so
improving the quality of the EM data.
ASPECTS OF THE CURRENT INVENTION
[0050] A first aspect the present invention refers to a bucked
transmitter, namely a transmitter which has its magnetic field
substantially annulled over a bucked volume. Bucking is done with a
field created by multiple bucking loops belonging to a bucking loop
arrangement, so the field may be annulled over a volume to a
greater degree than can be accomplished with a single bucking
loop.
[0051] The bucked transmitter comprises the bucking loop
arrangement and a transmitter sending a known current waveform into
a transmitter loop mounted on a substantially rigid framework. The
bucking loop arrangement comprises a plurality of separate,
electrically conducting bucking loops mounted on a bucking loop
support, so that the bucking loops are held in place with respect
to each other, and substantially in-place with respect to the
transmitter loop. Currents that are energized in the bucking loops,
with a current controller having a waveform substantially similar
to the transmitter waveform, create a bucking magnetic field which
substantially annuls the primary magnetic field over a bucked
volume. In one variation, this current controller may refer to the
transmitter, with the transmitter and bucking loops in series,
while in other variations, the each loop may be driven by a
separate current controller. In any variation, the bucking magnetic
field is substantially in a direction opposite to, and is
substantially equal in magnitude with, the primary magnetic field
within the bucked volume, such that the bucking magnetic field
substantially annuls said primary magnetic field over a larger
volume than could be done with a single loop. The transmitter
currents and the bucking currents may be sensed with current
monitors, with the currents recorded using a data recording means,
such as a data acquisition computer. The substantially rigid
transmitter loop framework may permit small motions of the
transmitter loop from its nominal location, but is sufficiently
rigid that the bucked volume is not displaced by a significant
distance in comparison to its dimensions.
[0052] In a second aspect of the invention, related to field of
electromagnetic surveying, an electromagnetic measurement apparatus
comprises a bucked transmitter, a magnetic field sensor, and a
receiver adapted for controlling said magnetic field sensor so as
to permit the output of said sensor to be measured and recorded.
The magnetic field sensor is located in the bucked volume, and is
mounted on a sensor support frame comprising a mechanical support
apparatus adapted to support and limit the motion of the sensor to
within the bucked volume. The sensor support frame and the bucking
loop support are connected to the substantially rigid transmitter
loop framework. The magnetic field within the bucked volume,
comprising the primary and bucking magnetic fields, may also
comprise a component that is scattered by the primary field due to
induction in the Earth. As the magnetic field sensor operates in a
volume where the primary field is bucked by the secondary field, it
may be operated at a finer resolution than would be otherwise
possible, and so may be adapted for measuring the component of the
magnetic field which may be scattered from the Earth.
[0053] In a third aspect of the invention, a mobile electromagnetic
measurement apparatus comprises an electromagnetic measurement
apparatus and a carrier to transport it. The mobile electromagnetic
measurement apparatus may also comprise a means of transporting the
carrier, a sensor to detect the position and orientation of the
carrier such as a global positioning system, and a means of
recording data, such as a data acquisition computer. For example,
variations of this aspect of the present invention may comprise
electromagnetic prospecting systems.
[0054] In fourth aspect of the current invention, the invention may
comprise part of an active airborne electromagnetic system, in
which the transmitter loop may be adapted to energize eddy currents
within the Earth, the magnetic field sensor may be adapted to
measure those eddy currents, and the bucking loops may be adapted
to cancel the primary field of the transmitter loop on the magnetic
field sensor without appreciably affecting either the eddy current
distribution in the Earth or the sensitivity of the magnetic field
sensor to the eddy current distribution in the Earth.
[0055] In a fifth aspect of the invention, the aforementioned
mobile electromagnetic measurement apparatus may adapted for towing
beneath a helicopter to comprise part of an airborne
electromagnetic prospecting system.
[0056] In another aspect of the current invention in which it is
employed in an AEM system, the bucking loops may have the same
orientation as, and lie coaxially with, the transmitter loop, but
with a smaller radius. Each loop may lie in a different plane,
axially offset from each other, as exemplified by a
pseudo-Helmholtz bucking circuit arrangement, or within the same
plane, as exemplified by the coplanar bucking circuit
arrangements.
[0057] In another aspect of the current invention, by bucking the
primary field of a transmitter loop over a larger volume than can a
single bucking loop, the invention so reduces the sensitivity of
the bucking to some deviations in loop geometry, and so provides a
more robust solution to bucking the primary magnetic field of a
transmitter antenna whose geometry may vary. Such variations may be
common in AEM systems in which a flexible transmitter loop is
employed. Provided the transmitter loop is substantially rigid, an
aspect of the current invention permits the annulling the primary
magnetic field over a larger volume and so may better accommodate
variations in transmitter loop geometry than is provided by the
current state of the art.
[0058] In another aspect of the current invention, the electrical
connections between the bucking loops and between the bucking loops
and the transmitter loop in the case of active bucking may be made
by way of either a coaxial or a twisted bifilar cable so that the
magnetic fields of the currents in these connections substantially
cancel each other.
[0059] In another aspect of the invention, a plurality of sensors
may be used to detect both the currents in the loops and the
relative locations of the loops with respect to each other,
permitting the geometry of the loops to be known in cases where the
loops are not nearly rigid, or are not nearly rigidly affixed to
one another, so as to permit the magnetic field, absent bucking to
be computed. Such sensors may consist of Hall effect current
monitors or equivalent detectors for detecting currents, and
devices such as cameras, AHRS (attitude, heading and reference
systems), differential positioning systems, laser rangers, radars,
strain gauges and sensors of equivalent function which may be used
to detect the relative changes in loop geometry. The primary and
bucking magnetic fields may thus be computed in real time and the
results recorded, or the aforementioned sensor data may be
recorded, so as to permit the magnetic fields to be computed at a
later time.
[0060] In all foregoing aspects of the current invention, the
transmitting loop and bucking loop geometries, and the respective
currents may be precisely measured and recorded so as to permit
accurate computation of the corresponding magnetic fields so as to
yield an improved separation of primary and scattered fields.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIG. 1 illustrates aspects of a three-loop coplanar bucking
configuration.
[0062] FIG. 2 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.15 meters on the
transmitter axis.
[0063] FIG. 3 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.25 meters.
[0064] FIG. 4 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.35 meters with
larger loops.
[0065] FIG. 5 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.15 meters with
smaller loops.
[0066] FIG. 6 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.25 meters.
[0067] FIG. 7 illustrates a comparison between the bucking achieved
by a three-loop coplanar bucking configuration and a two-loop
coplanar bucking configuration over a range of 0.35 meters with
larger loops.
[0068] FIG. 8 illustrates aspects of a pseudo-Helmholtz
configuration in plan.
[0069] FIG. 9 illustrates aspects of a pseudo-Helmholtz
configuration in section.
[0070] FIG. 10 illustrates aspects of a bucking loops mounted on a
sphere
[0071] FIG. 11 illustrates a comparison between the bucking
achieved by a three-loop pseudo-Helmholtz bucking configuration and
a two-loop coplanar bucking configuration over a range of 0.25
meters.
[0072] FIG. 12 illustrates a comparison plot between a three-loop
pseudo-Helmholtz bucking configuration and a two-loop coplanar
bucking configuration over a range of 0.25 meters with larger
loops.
[0073] FIG. 13 illustrates a comparison between the bucking
achieved by a three-loop pseudo-Helmholtz bucking configuration and
a two-loop coplanar bucking configuration over a range of 0.4
meters.
[0074] FIG. 14 illustrates a comparison between the bucking
achieved by a three-loop pseudo-Helmholtz bucking configuration and
a two-loop coplanar bucking configuration over a range of 0.15
meters with smaller loops.
[0075] FIG. 15 illustrates aspects of an electrical circuit for
driving of a 3-loop bucking configuration.
[0076] FIG. 16 illustrates aspects of another electrical circuit
for driving of a 3-loop bucking configuration.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE CURRENT INVENTION
[0077] In the preferred embodiment of the present invention, the
invention comprises a bucked transmitter mounted on a substantially
rigid framework, with a transmitting electrical circuit comprising
a substantially circular transmitter loop supported by said
substantially rigid framework, a transmitter adapted to energize
said transmitting circuit with a transmitting electric current
having a known waveform so as to create a primary magnetic field,
and a bucking loop assembly, comprising a bucking loop support to
which the bucking loops are attached, and which is stably located
with respect to geometrical aspects of the primary magnetic field.
The bucking loop assembly is suitable for conducting a current that
forms a bucking magnetic field which substantially annuls the
primary magnetic field within a bucked volume. The primary magnetic
field has a time variation substantially similar to the time
variation of the transmitting electric current, with the
geometrical aspects of the primary magnetic field being provided by
the geometry of the transmitter loop. The conducting loops of the
bucking loop assembly are energized with electric current to create
a bucking magnetic field with known geometrical and time variation,
wherein the bucking magnetic field is substantially in a direction
opposite to, and is substantially equal in magnitude with, the
primary magnetic field within the bucked volume. Accordingly,
within the bucked volume, said bucking magnetic field substantially
annuls the primary magnetic field.
[0078] The preferred embodiment comprises a substantially planar
transmitter loop in which the bucking loop assembly is mounted in a
coplanar configuration. Each loop in the bucking loop assembly is
substantially coaxial with the transmitter loop such that the
bucking loop axes are substantially aligned with the axis of the
transmitter loop, with each bucking loop being smaller than the
transmitter loop. The bucking loops are mounted to inhibit motion
relative to the transmitter loop and the bucked volume. The bucked
transmitter is configured with one electrical circuit comprising
the transmitter and bucking loops in a series configuration.
Connections between the transmitter and bucking loops are formed
with coaxial or twisted bifilar conductors to suppress the magnetic
fields of the current in the conductors connecting the loops.
[0079] In the preferred embodiment, the aforementioned bucked
electromagnetic transmitter comprises part of a helicopter-borne
electromagnetic surveying system. The bucked electromagnetic
transmitter is mounted on a carrier towed below helicopter, The
transmitter loop is substantially horizontal, and high-precision
magnetometers, located in the bucked volume, are used to sample the
magnetic field within the 1-10,000 Hz acquisition band. The
magnetometers are mounted on a sensor support frame comprising a
mechanical support apparatus adapted to support and limit the
motion of the magnetic field sensor to the bucked volume of the
bucked transmitter, such that sensor support frame is affixed to
the transmitter loop framework of the bucked transmitter. A data
acquisition system records the transmitter current, the sensed
magnetic field, the attitude of the sensors, and the system
geolocation as the helicopter towing the system traverses the
Earth. The data acquisition system may also record variations in
the relative geometry of the transmitter loop, bucking loops and
the sensors. The geolocation system may comprise a means to record
the position and altitude of the carrier with respect to the
Earth's geographical coordinate system, such as a global position
system (GPS) and an attitude, heading and reference system
(AHRS).
[0080] The bucked volume in which the magnetic field sensor may
move is defined by a locus of null points which lie approximately
on the axis of the transmitter loop where the field is annulled.
This locus extends bidirectionally from the plane of the
transmitter loop to a nulling distance where the bucking field
fails to substantially annul the primary field. The bucked volume
has a scale length which is approximately governed by the length of
the nulling axis, and which is approximately centered on the point
where the axis of the transmitter loop and the plane of the
transmitter intersect.
[0081] FIG. 1 illustrates a plan view of the bucked transmitter of
the preferred embodiment, in which a three-loop coplanar
configuration is used. In this embodiment, an outer, substantially
circular conductive loop, 1, comprising of N0 turns of wire, where
N0 may be 10, is wrapped in a substantially circular manner around
a center point at a radius, R0, which may be 12.5 meters. A second
substantially circular loop of wire, 2, comprising N1 turns, where
N1 may be 4, is wound in the opposite sense to the outer loop 1, at
a radius R1, which may be 3.0373 meters. A substantially circular
inner loop, 3, comprising N2 turns, which may be 1 turn, is wound
in the same direction as outer loop 1, at a radius R2 which may be
1.9344 meters. In FIG. 1, arrows surrounded by ellipses illustrate
a possible selection of winding directions, and by implication, the
direction of electric current flow. However, a person skilled in
the art will understand all windings may be in the sense opposite
to the ones shown with equal effect, by reversing the direction of
the current, and will understand that the numbers of turns and
radii of the bucking loops may be adapted to accommodate a
different number of turns and radius of the transmitter loop.
[0082] In FIG. 1, zone 4 defines the location where the primary
field of the transmitter loop 1 is substantially annulled by the
bucking loops 2 and 3. The center point of zone 4 defines the
reference point of the invention, and lies on the axes of the
transmitter and bucking loops in the plane of the transmitter loop.
A magnetic field sensor able to move within zone 4 may be used to
acquire high-sensitivity magnetic measurements, while avoiding
saturation from the large primary magnetic field and while also
reducing variations in the recorded magnetic field resulting from
large changes of the primary magnetic field in the bucked
volume.
[0083] Again referring to FIG. 1, the primary field is created by
currents in outer loop 1, which serves as the transmitter loop.
Outer loop 1 is held in place relative to the bucking loops and the
nulling zone in as rigid manner as possible, but due to its large
radius, small deviations in the position of loop 1 are less
important to the performance of the invention than are deviations
in the locations of loops 2 and 3.
[0084] An electrical connection, 5, permits a series connection of
loop 1 and bucking loop 2, and a second electrical connection, 6,
permits the series connection of bucking loop 2 and bucking loop 3.
Electrical connections 5 and 6 are disposed to minimize the
magnetic field of the electrical currents they conduct, and may be
formed from a twisted bifilar, coaxial or other equivalent
structure where the external magnetic fields from oppositely
flowing electrical currents are mutually cancelled. A person
skilled in the art will understand that the order of the loops in
the aforementioned series circuit may be altered with little effect
on the invention where the transmitter current has a low time
variation such that the circuit capacitances are unimportant.
[0085] FIG. 2 illustrates a comparison the z-component of the
magnetic field along the z-axis of rotational symmetry through the
center of the loops of the preferred embodiment. A standard 2-loop
coplanar nulling is compared to the aforementioned 3-loop case of
the preferred embodiment for a 288 Amp current in the turns of the
loops. In the preferred embodiment, the magnetic field is
compensated to better than 1 part in 10,000 over a +/-0.15 meter
range. The fractional reduction in the transmitter moment,
indicated as FrMr, is negligible. In this example, R0=12.5 meters,
R1=3.0733 meters, R2=1.9344 meters, N0=10, N1=4 and N2=1.
[0086] FIG. 3 illustrates the comparison the z-component of the
magnetic field in another embodiment of a three-loop coplanar
configuration. In this embodiment, the primary field of a 14-turn
transmitter loop is nulled to 2 parts in 10,000 over a +/-0.25
meter range, R0=12.5 meters, R1=2.1526 meters, R2=1.3545 meters,
N1=4 and N2=1.
[0087] FIG. 4 illustrates the comparison the z-component of the
magnetic field in another embodiment of a three-loop coplanar
configuration. In this embodiment, the primary field of a 10-turn
transmitter loop is nulled to 2 parts in 10,000 over a +/-0.25
meter range, R0=12.5 meters, R1=4.1572 meters, R2=2.483 meters,
N1=5 and N2=1. In this embodiment, larger diameter compensation
loops are used.
[0088] FIG. 5 illustrates the comparison the z-component of the
magnetic field in another embodiment of a three-loop coplanar
configuration. In this embodiment, the primary field of a 14-turn
transmitter loop is nulled over a +/-0.15 meter range, R0=12.5
meters, R1=2.1591 meters, R2=1.3649 meters, N1=4 and N2=1. In this
embodiment, smaller diameter bucking loops are employed, resulting
in a smaller volume of accurate nulling.
[0089] FIG. 6 illustrates the comparison the z-component of the
magnetic field in another embodiment of a three-loop coplanar
configuration. In this embodiment, the primary field of a 14-turn
transmitter loop is nulled with R0=12.5 meters, R1=2.1526 meters,
R2=1.3545 meters, N1=4 and N2=1. This embodiment permits a wider
range of nulling along the z-axis of the loop system, and by
implication, radially as well.
[0090] FIG. 7 illustrates the comparison the z-component of the
magnetic field in another embodiment of a three-loop coplanar
configuration. In this embodiment, the primary field of a 14-turn
transmitter loop is nulled with R0=12.5 meters, R1=2.9425 meters,
R2=1.7262 meters, N1=5 and N2=1. In this embodiment, the bucking
loops are enlarged to give a larger region of accurate
compensation.
[0091] While the foregoing examples demonstrate various embodiments
possible with coplanar bucking loops, these examples are meant to
be illustrative of possible embodiments of the present invention,
and not to be interpreted to limit the scope of the invention, for
example, to the number of loops, numbers of turns, or number of
radii provided in those examples. For example, embodiments of the
present invention may comprise a bucking loop assembly formed in
the pseudo-Helmholtz configuration, as illustrated below.
[0092] FIG. 8 illustrates the plan view of a pseudo-Helmholtz
embodiment of the current invention which is implemented with a
single pair of bucking loops in the active mode. Transmitter loop 1
creates a primary field which is bucked in volume 4 by the pair of
bucking loops 7. The sense of the current in the bucking loops is
opposite to that in the transmitter loop. Transmitter loop 1 is
substantially circular with a radius denoted R0, while the bucking
loops are coaxial with the transmitter loop with radius denoted R1,
but offset from the plane of the transmitter loop by distances
+/-Z1 parallel to the z-axis of symmetry of the transmitter loop,
as illustrated in FIG. 9. Loops 1 and 7 are connected by electrical
cable 8 to carry current between the transmitter and bucking loops.
Electrical cable 8 may drive both bucking loops in parallel, or in
series from one loop to the other. Electrical cable 8 is fashioned
in a such manner as to create a minimal magnetic field, such as may
be obtained using a coaxial, twisted bifilar or other geometry with
similar effect.
[0093] As in the case of the preferred embodiment, zone 4 defines
the location where the primary field of the transmitter loop 1 is
substantially annulled by the bucking loops 7. The center point of
zone 4 defines the reference point of the invention. A magnetic
field sensor able to move within zone 4, may be used to acquire
high-sensitivity magnetic measurements, while avoiding saturation
from the large primary magnetic field and while also avoiding
significant changes in the recorded magnetic field resulting from
variations of the primary magnetic field in the bucked volume.
[0094] Again referring to FIG. 9, the primary field is created by
current in outer loop 1, which serves as the transmitter loop.
Outer loop 1 is held in place relative to the bucking loops and the
nulling zone in as rigid manner as possible, but due to its large
radius, small deviations in the position of loop 1 are less
important to the performance of the invention than are deviations
in the locations of loops 7.
[0095] FIG. 10 illustrates, in section view, a bucking loop
configuration implemented with coaxial loops having differing
radii. In this example, pairs of Helmholtz loops (for example 110
and 114), together with a single coplanar bucking loop 112, are
wound on a spherical shell 100. The multiple bucking loops, 110 . .
. 114 are arranged to annul the component of the primary field in
region 120 parallel to the axes of the loops using means analogous
to the pseudo-Helmholtz style illustrated in FIGS. 8 and 9. In this
embodiment of the invention, a plurality of loops may be employed
to substantially buck the primary field in region 120 which may be
larger than the bucked volume achieved with fewer bucking loops
[0096] FIG. 11 illustrates the effect which may be obtained from an
embodiment of the invention using pseudo-Helmholtz bucking. This
embodiment is implemented with two bucking loops as illustrated in
FIGS. 8 and 9. The transmitter loop is substantially circular with
a radius of 12.5 meters and 10 turns. Each bucking loop is
substantially circular and wound with one turn with a radius of
1.7956 meters, and offset from the plane of the transmitter loop by
0.8922 meters. In this embodiment of the current invention, the
primary field is annulled to better than 1 part in 10,000 over
+/-0.25 meters.
[0097] FIG. 12 illustrates the effect which may be obtained from a
pseudo-Helmholtz embodiment where the transmitter loop is
substantially circular with a radius of 12.5 meters and 10 turns.
Each bucking loop is substantially circular and wound with two
turns having a radius of 3.7187 meters, and offset from the plane
of the transmitter loop by 1.7371 meters. In this embodiment of the
current invention, the primary field is annulled over a larger
volume than in the previous case, due to the larger bucking loop
set employed.
[0098] FIG. 13 also shows the effect which may be obtained from
another pseudo-Helmholtz embodiment of the current invention where
the transmitter loop is substantially circular with a radius of
12.5 meters and 14 turns. In this embodiment, substantial bucking
is obtained over a distance of 0.4 meters on the z-axis. Each
bucking loop is wound with 2 turns on a radius of 2.5886 meters and
a z-offset of 1.2665 meters.
[0099] FIG. 14 also illustrates the effect which may be obtained
from another pseudo- Helmholtz embodiment where the transmitter
loop is substantially circular with a radius of 12.5 meters and 14
turns. In this case, a compact pseudo-Helmholtz loop set is used to
achieve excellent cancellation of the primary field over a distance
of +/-0.15 meters on the z-axis. In this embodiment of the current
invention, the bucking loops each have one turn, and are wound on a
radius of 1.2786 meters with a z-offset of +/-0.63861 meters.
[0100] While the forgoing examples of embodiments of the current
invention, illustrate the effect of bucking with a 2-loop bucking
loop assembly, these examples are meant to demonstrate possible
embodiments the current invention, and are not meant to imply a
restriction on the numbers of loops employed, their offsets,
numbers of turns or their radii. Some embodiments of the current
invention may employ, by way of example, combinations of coplanar
and pseudo-Helmholtz configurations, combinations of coplanar
configurations, some of which may be offset from the plane of the
transmitter loop, or combinations of pseudo-Helmholtz loops offset
at different radii.
[0101] Other embodiments of the current invention may employ the
aforementioned combinations of pseudo-Helmholtz bucking loops by
rigidly attaching them to the same structure as supports the
sensor.
[0102] In other embodiments of the current invention, the bucked
volume may be offset from the plane or from the axis of the
transmitter loop by so arranging the bucking loops to substantially
annul the primary magnetic field in a volume that is displaced
either from the plane of the transmitter loop, its axis, or in
general at any point in space. Such an embodiment may be useful in
AEM surveying to bucking the primary in the vicinity of a magnetic
field sensor mounted on a tow cable and which is accordingly offset
from the axis of symmetry of a towed transmitter. In such
embodiments, the turns and diameters of the bucking coils will have
to be chosen to cancel the gradient of the primary field in the
bucking volume.
[0103] FIG. 15 illustrates various aspects of the electric circuits
in an embodiment of the current invention comprising an active
source electromagnetic system. Power supply 300 energizes
transmitter 301 through electrical cable 302. Transmitter 301 forms
a current waveform which is output on electrical cable 304 to form
a series circuit with transmitter loop 305, first bucking loop 306
and second bucking loop 307. Sections of electrical cable 304 may
comprise coaxial, twisted bifilar or any such conductor geometry so
as to suppress magnetic fields from the bidirectional current
flowing within, as noted in aforementioned example embodiments.
[0104] FIG. 16 illustrates alternative aspects of the electric
circuits in an embodiment of the current invention, for the cases
when it is implemented as part of an active source electromagnetic
system. In FIG. 16, the bucking loops are in a separate circuit
304a from circuit 304b which contains the transmitter loop 305.
Each circuit is driven by separate current controller 301a, and
301b, of which 301b comprises the transmitter, with each controller
energized by currents on their respective supply cables, 302a and
302b. Both controllers provide identical current waveforms to their
respective loops.
[0105] While this invention has been described in conjunction with
the exemplary aspects outlined above, various alternatives,
modifications, variations, improvements, and/or substantial
equivalents, whether known or that are or may be presently
unforeseen, may become apparent to those having at least ordinary
skill in the art.
[0106] Accordingly, the exemplary aspects of the invention, as set
forth above, are intended to be illustrative, not limiting. Various
changes may be made without departing from the spirit and scope of
the invention. Therefore, the invention is intended to embrace all
known or later- developed alternatives, modifications, variations,
improvements, and/or substantial equivalents.
* * * * *